This is already programmed into koreader, you can use text, image, or directory with images for both suspend and power off.
Here you can found instructions:
https://github.com/koreader/koreader/pull/2349
Here is how to do with images:
Quote:
["suspend_screensaver"] = {
["path"] = "/mnt/onboard/.screensaver/sleeping"
},
["poweroff_screensaver"] = {
["path"] = "/mnt/onboard/.screensaver/powered_off"
},
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You either point it to image file, or directory where images are located.
You put this into settings.reader.lua file. But, to be safe, made backup before editing settings.reader.lua file because if you use wrong syntax file will be deleted and you'll use all your settings. So saving it before editing is best bet not to have problems.
On thread there you have more informations, so read all posts there if you're interested in more info.